Underground piping systems are essential in providing the transportation of liquids and gases to homes and businesses. Used mostly by utilities in sewer pipes, water pipes, water mains, gas mains, and other applications, such pipes are often found many feet underground or in inaccessible areas, such as under buildings or roadways.
Due to cyclical loadings, premature wear, manufacturing defects, corrosion, and other factors, these pipes can often develop ruptured or weakened areas requiring repair. Although the surest way to repair such leaks is to replace the damaged section, replacement is often difficult and expensive.
Recently, in situ pipe repair procedures have been developed which include the insertion of a pliable polyester felt sleeve impregnated with a thermosetting resin. The sleeve is inserted coaxially through the damaged pipe portion and pressurized so that the resin-impregnated sleeve presses firmly against the inner wall of the damaged pipe. The expanded liner is then permitted to cure to form a new lining within the original pipe.
In Europe, where fiberglass liners have been developed, higher strengths have been provided without the use of polyester, by simply impregnating glass fiber laminates with synthetic resin.
Glass fiber mats are attractive replacements for polyester tubing material for making pipe repairs since they can achieve the same mechanical properties with less than half the wall thickness of comparable polyester liners. Unfortunately, since only polyester tubing has been contemplated, many United States pipe repair specifications call for a minimum liner thickness requirement for different circumstances. These thicknesses vary between 6 and 18 mils. Manufacturing an all glass liner within these tolerances would be wasteful, not to mention costly, since only about two mills of glass fabric is necessary to provide sufficient tensile strength for an underground pipe repair.
There appears to be some recognition of a weakness along the longitudinal seams of current conduit liners made from either polyester or glass fiber. Since most of these liners are made from folded mats which are joined with a longitudinal butt-seam for bonding the opposite lateral edges of the mat, a discontinuity in the liner's sidewall is created which is significantly weaker than the remaining cross-section at any other point along the circumference of the liner. This discontinuity presents a threat of delamination or separation during insertion and pressurized expansion of the liner inside a pipe.
Efforts to overcome this weakness in the wall structure have included laminating or sewing a polyester patch to the seam of polyester liners. Although this effort to reinforce an apparent weakness in the side wall has been somewhat successful, it results in a distortion of the smooth circumference of the liner, making it rather difficult to match the inner diameter of the pipe to be repaired. This technique has also not been very successful with glass liners since the irregular texture of the fiberglass resists efforts to create a sound joint. Accordingly, fiberglass liners are usually butt-joined together by a longitudinal stitch or line of adhesive, leaving a weak site through the entire cross-section of the liner.